The invention concerns an apparatus for reactively coating a substrate, with silicon dioxide (SiO.sub.2) for example. It comprises a power supply which is connected to an electrode disposed in an evacuable coating chamber. This electrode is electrically connected to a target which is sputtered. The sputtered articles are deposited on the substrate while a process gas and a reactive gas are supplied to the coating chamber.
In known processes for coating substrates by means of cathode sputtering where materials with a high affinity for the reactive gas are used, the problem arises that not only the substrate but also parts of the apparatus like the inner wall of the process chamber or parts of diaphragms are coated with materials of no or poor electrical conductivity. This requires a frequent change of process parameters during one single coating procedure or even a frequent interruption of the process and even a frequent cleaning or replacement of parts of the apparatus. In particular, it leads to highly undesired electric discharges (arcings).
The present invention provides an apparatus for sputtering materials with a high affinity for the reactive gas. This process should be stable and free of arcings and render a cleaning of parts superfluous. Conventional and presently existing apparatus, however, should also be suitable for this purpose without requiring essential or expensive modifications and changes. Moreover, the apparatus should operate trouble-free especially over longer periods even when insulating layers like SiO.sub.2, Al.sub.2 O.sub.3, NiSi.sub.2 -oxide, ZrO.sub.2, TiO.sub.2, ZnO, Ta.sub.2 O.sub.3, SnO.sub.2 are reactively deposited.
This object is accomplished in the invention by providing two electrodes which are electrically insulated from one another and from the sputtering chamber. One of the electrodes is a magnetron cathode where the cathode base and the target material are electrically connected to one another. The other electrode functions as an anode in the plasma discharge. Provision is made for a power supply for direct current which has an electrically floating output. The negative pole thereof, with a throttle being interposed is connected to the cathode and its positive pole is connected to the anode. A first low-induction, HF-suitable capacitor is interposed between the cathode and the anode, and a second low-induction, HF-suitable capacitor is interposed between the anode and the first electrically insulated vacuum chamber.
An alternative embodiment of the invention has a cathode with an annular or oval target. This cathode is electrically insulated from the vacuum chamber and the anode. It is configured as a magnetron cathode and is composed of two parts which are electrically insulated from one another. The target base inclusive of yoke and magnet is the one part. It is connected to the negative pole of a DC power supply with a capacitor being interposed. The target is the other part and it is connected to the DC power supply via a line where a throttle and a resistor parallel thereto are interposed. Via another capacitor, the target is connected to the positive pole of the power supply and to the anode which in turn is connected to ground. Interposed therein are a capacitor and a resistor downstream of it.